Permanent magnet (PM) brushless dynamoelectric machines have the highest power density in comparison with all other classical electrical machines. They also have very high efficiency and good dynamic performance. On the other hand PM brushless machines exhibit constant magnetic flux which, conventionally mandates use of an external solid state converter to maintain control. As a result, constant magnetic flux requirements may limit utilization of PM brushless machines in applications as generators when the prime mover speed is variable, e.g., aircraft generators. In addition, the constant magnetic flux also limits their utilization as variable-speed motors for selected applications, e.g., electric or hybrid-electric vehicle.
Control of magnetic excitation flux is not available in standard PM brushless dynamoelectric machines. Such control has previously only been practical with current vector control techniques. In vector control, a motor controller operates as an inverter system to inject the direct axis (d-axis) current that weakens the PM flux to a desired degree. However, such d-axis current injection to control magnetic flux excitation has certain drawbacks, such as a significant increase in stator winding losses that can result in excess heat dissipated in the stator winding and irreversible demagnetization of low energy density rotor PMs, such as rotor PMs of the ferrite type.
Electrical power generation systems powered by variable speed prime movers that require highly regulated electrical output, such as electrical power generation systems used for aeronautical applications, generally use a wound field synchronous machine (WFSM) that serves as an electrical generator. Electrical power generation systems may alternatively employ an electrical machine of the PM type as an electrical generator. Such a PM machine is capable of much higher angular velocity than a WFSM of similar output and, therefore, is capable of direct coupling to the prime mover, thereby potentially eliminating the reduction gearbox typically employed. This results in reduced weight, cost, and complexity of an electrical power generation system. However, traditional PM machines have no convenient means to alter magnetic flux for regulating their output. Therefore, it would be advantageous to have a convenient means to modify and regulate magnetic flux in a PM dynamoelectric machine.